1. Field of the Invention
The present invention concerns a process for geophysical exploration by means of measuring the propagation of vibrations in the ground, which is called seismic measuring.
2. The Prior Art
The known principle for seismic exploration consists in creating, through a violent shock such as an explosion, a disturbance which is propogated through the ground at variable speeds depending on the nature of the terrain traversed.
In particular, the disturbances are reflected and refracted when they reach the interfaces between layers of terrain whose propagation speeds are different.
Many seismic vibration collectors are distributed at appropriate locations to detect the arrival of the disturbances and thus gather information on the nature of the terrain below the surface of the ground.
These methods are in particular applicable to the exploration of subsoil covered by the sea, and in this case, the disturbance sources are placed below seal level (for example a few meters deep); the disturbances are propagated to the bottom and are transmitted to the interior of the ground where they encounter obstacles or successive layers of terrain which cause refractions and reflections; the reflected or refracted disturbances return and are propagated through the water to the surface where they are detected by hydrophones (vibration collectors) placed slightly below the surface of the water.
There is a particular problem in marine seismic exploration when the sea bottom has a propagation speed characteristic very different from that of the propagation speed in water. The latter is practically always equal to 1500 meters per second, and one often finds sea bottoms with much greater speeds. The disturbances emitted in water at a few meters depth are propagated to the bottom; a portion penetrates by refraction into the ground, but the remainder is reflected and starts again towards the surface where it is detected by the hydrophones.
Not only are these waves not useful because they bear no information on the nature of the subsoil, but also they interfere with the useful waves which have penetrated into the ground and have come out of it after reflection, thus confusing the seismic recordings, the utilization of which is already very complex.
In particular, all the disturbance waves arriving at the bottom with an incidence greater than the total angle of refraction are integrally reflected upwardly.
According to the laws of refraction, the critical angle of total reflection taken in relation to the perpendicular to the surface of the sea bottom (vertical) is that where the sine is equal to the relation between the propagation speed in water (propagation medium for the incident wave) and the propagation speed in the superficial layer of the sea bottom (propagation medium of the refracted wave).
For a sea bottom speed of 3000 m/s, the incidence of total reflection is only 30 degrees. For a faster speed, it is even smaller, and all the disturbance waves emitted with a greater incidence are a total loss and, through their reflections on the bottom and their refraction along this bottom, they spoil the utilization of the useful seismic signals.
The usual seismic sources are omnidirectional and the energy they emit outside of the vertical cone from angle to summit equal to the critical incidence of total reflection is lost.